In a large-scale computer network, to ensure reliability of data, there is a case where data is multiplexed and stored in a plurality of storage devices. As a technique of multiplexing data, there has been proposed a method of distributing and storing data in a plurality of storage devices which are respectively connected to a plurality of nodes (disk nodes) on a network. A system in which data are thus distributed and managed in a plurality of nodes on the network is referred to as a multinode storage system.
In the multinode storage system, a virtual disk is defined in a node for access (access node), and data is input/output via the virtual disk. A storage area in the virtual disk is divided into a plurality of segments, and each of the segments is associated with a plurality of storage areas within the respective storage devices. When an application specifies a segment on the virtual disk to write data therein, the data is written in respective storage areas of the storage devices, associated with the specified segment. By multiplexing data as described above, it is possible to improve the reliability of the data (see e.g. International Publication Pamphlet No. WO2004/104845).
By the way, if data is duplexed, in the storage devices, storage areas are used which correspond to twice as large as the data amount to be stored. Therefore, also to reduce waste of resources, data which does not require reliability is often controlled not to be duplexed. However, if a computer system has been operated for a long term, also as to data supposed not to require high reliability, it sometimes becomes necessary to ensure reliability of the data by duplicating the same. In this case, it is necessary to transfer the data which has been stored in a storage device to a virtual disk for duplexing.
To duplex the data stored in the storage device (used disk), using the virtual disk, there are two methods of transferring data described hereafter.
First, if access to the used disk can be stopped, it is possible to make use of the following data transfer method:    After creating a virtual disk, access to the used disk is suspended.    Data stored in the used disk is copied to the virtual disk.
However, in the above-described data transfer method, it is impossible to access the used disk during copying, and hence service stop time occurs due to the data transfer. If it is not possible to stop access to the used disk, copying of the data is carried out by the following data transfer process:    After creating a virtual disk, a snapshot of the used disk is created by the OS (Operating System) in use. Snapshot is a technique for virtually duplicating data of the whole disk at a certain time point. For example, according to a copy-on-write snapshot, before creating a snapshot, the relationship between data and a block in the disk storing the data is recorded. Then, the access to the disk is monitored, and if data is updated, the data before the update is copied to another area (shadow copy area). This makes it possible to preserve in the disk the data at the time of creating the snapshot.    The snapshot is copied to the virtual disk by the OS in use. At this time, in place of data which has been updated after creating the snapshot, corresponding data before the update is read out from the shadow copy area, whereas data which has not been updated is read out from the used disk. The data which has been copied to the virtual disk is actually written in the storage areas of the disk devices, which are associated with the segments which form the virtual disk.    Access to the used disk is stopped, and the used disk is synchronized with the virtual disk. That is, the data which has been updated after creating the snapshot of the used disk is copied to the virtual disk. This causes the contents of the used disk and the contents of the virtual disk to become identical to each other.    Access to the virtual disk is started.
However, the two above-described data transfer techniques suffer from the following problem:
Both the two data transfer methods require a copying time proportional to the disk capacity. Therefore, it takes a longer time to transfer data stored in a disk device having a larger capacity. If the copying time is longer, extra processing load is placed on the computer to which the used disk is connected, resulting in a reduced processing efficiency of the entire system.